A method which has been used to increase the capacity of cellular communication systems is the concept of hierarchical cells wherein a macro-cell layer is underlayed by a layer of typically smaller cells having coverage areas within the coverage area of the macro-cell. In this way, smaller cells, known as micro-cells or pico-cells (or even femto-cells), are located within larger macro-cells. The micro-cells and pico-cells have much smaller coverage thereby allowing a much closer reuse of resources. Frequently, the macro-cells are used to provide coverage over a large area, and micro-cells and pico-cells are used to provide additional capacity in e.g. densely populated areas and hotspots. Furthermore, pico-cells can also be used to provide coverage in specific locations such as within a residential home or office.
In order to efficiently exploit the additional resource, it is important that handover/cell selection performance between the macro-cell layer and the underlying layer is optimized.
The current trend is towards introducing a large number of pico-cells to 3G systems. For example, it is envisaged that residential access points may be deployed having only a target coverage area of a single residential dwelling or house. The use of residential cells may not only provide increased capacity but may also facilitate service and subscription differentiation. For example, a subscriber may pay a substantially lower cost when at home using his dedicated residential access point than when using the cellular communication system remotely. A widespread introduction of residential access points would result in a very large number of small underlay cells within a single macro-cell.
However, underlaying a macrolayer of a 3G network with a pico-cell (or micro-cell) layer creates several issues that must be addressed. In particular, it makes efficient handover/cell selection techniques even more critical. In particular, it is desirable that handover/cell selection is efficient and simple and preferably allows seamless mobility for the mobile station between the layers. Furthermore, it is desired that the interference from the underlay layer to the macrolayer is minimized.
However, such requirements are not always easily met and may in many cases be in conflict with each other. For example, soft handover techniques are used in many CDMA systems to provide efficient handover with reduced interference. However, soft handover requires time synchronization of base stations and as the backhaul from e.g. residential access points typically cannot be guaranteed to be sufficiently time synchronized to support recombination, it is impractical to support soft handover for a residential access point underlay layer.
However, operating an underlay without soft handover tends to result in increased interference between the layers and specifically results in issues with managing the near far effects. In order to avoid this interlayer interference, it is advantageous to operate the macro-layer and the underlay layer on different CDMA frequencies thereby providing a high degree of separation between the layers.
However, using different frequencies for the different layers impacts the handover/cell selection operation of the systems. Specifically, for capacity and/or billing reasons, it is desirable that the underlay cell is used whenever possible even if a high quality of service could be provided by the macro layer. However, in systems such as UMTS, the handover algorithm is biased towards minimizing the number of handovers that are performed and the algorithm will tend not to handover a mobile station to a different frequency unless it is currently experiencing poor quality of service. Specifically, if a mobile station is receiving good coverage, the handover algorithm can choose to ignore a secondary frequency completely. Indeed, for CDMA same frequency measurements are substantially simpler to perform than measurements of other frequencies than the current serving frequency. Therefore, in many scenarios, mobile stations will not perform other frequency measurements unless the current conditions are poor. Therefore, a residential access point using a different frequency will not be detected by mobile stations in a good macro-cell coverage area thereby resulting in the mobile station remaining on the macro-cell rather than switching to the underlay cell.
Hence, an improved cellular communication system would be advantageous and in particular a system allowing improved handover/cell selection between macro-cells and underlay cells using different frequencies; reduced interlayer interference; improved mobility support; increased handover to underlay cells; facilitated operation; efficient implementation and/or improved performance would be advantageous.